Early Mars didn’t have enough CO2 to keep water liquid, Curiosity finds

Mars at the boundary between night and day. Gale Crater is the crater
with a mound inside it near the center of the image, beginning to catch
morning light. Northward is to the left. This view was created using
three-dimensional information from the Mars Orbiter Laser Altimeter,
which flew on NASA's Mars Global Surveyor orbiter.

NASA/JPL-Caltech

Even though Mars is currently too cold and has
too little pressure to prevent water from freezing, the planet had
liquid water on its surface in the distant past. That could happen if
the ancient Martian atmosphere, thicker than its present-day
counterpart, had enough greenhouse gasses to keep the planet warm and
the water liquid. So over the past decades of our observations and
robotic visitations of Mars, researchers have been looking for evidence
for Mars' past carbon dioxide levels.

The evidence we've gathered indicates there was some CO2 present
but not nearly enough to keep water liquid, especially given that the
early Sun was less active than it is at present. So far, these estimates
contained large uncertainties, so it remained possible that there was
enough carbon in the atmosphere to allow the ancient water to flow.

A team of researchers has created a new
estimate of Mars' ancient carbon levels using data collected by the
Curiosity rover. They've also concluded that there was nowhere near
enough CO2 to warm the planet to the point where water on the surface would remain liquid.

Curiosity

Enlarge/ A "selfie" taken by Curiosity (actually assembled from a mosaic of selfies). The rover captured the public's imagination with its daring, Rube Goldberg-ian skycrane touchdown on Mars.

NASA

During its trip towards Aeolis Mons (the peak
at the center of the Gale crater), Curiosity came upon what looked to be
the remains of an ancient lake. This was a region of about 70 meters
with mudstones, siltstones, and sandstones left behind by ancient water,
somewhere between 3.8 and 3.1 billion years ago. The rover analyzed
these materials as it passed, but there were no carbonate minerals,
which would have made assessing past carbon levels easier. However,
certain clays could provide a way to estimate the ancient carbon
concentration.

When these clays formed, CO2 would
have dissolved into their structure. While we can't measure them, we
can infer their presence indirectly, as dissolved carbon limits the
solubility of iron-bearing olivine. The amount of this olivine in the
clays puts a limit on the amount of CO2 that could be in the water that the clays formed in.

This technique is also used to estimate
Earth’s early atmosphere, but here it’s actually less reliable because
the presence of life contaminates the process. The Martian rocks have
been comparatively undisturbed, making them prime targets for science.
“In many ways, deriving [CO2]
estimates from Gale Crater sedimentary rocks is more straightforward
than doing so in their terrestrial equivalents,” the authors write in
their paper.

The researchers made a detailed analysis of
the compounds involved in the clays, comparing them to those found in
thousands of lakebeds on Earth. The time it takes for various
chemicals to mix in with the clays is an important factor as well, as it
affects how much of them we’d observe now.

Results

A simulated view of Gale Crater Lake, measuring
about 150km across, on Mars about 3 billion years ago.

NASA/JPL-Caltech/ESA/DLR/FU Berlin/MSSS

The science target: Gale Crater with its central peak, Mt. Sharp, in the center.

Image credit: NASA JPL

Gale Crater viewed from the south, with
Curiosity's landing area highlighted. Methane appears to be coming from
somewhere to the north.

The rim of Gale Crater, where the rover has been
active since landing in 2012, is visible in the distance, through the
dusty haze.

NASA/JPL-Caltech/MSSS

The researchers conclude that the Martian atmosphere at the time the clays were formed had a lot more CO2
than the lower limits of previous estimates. But the numbers are still
fall far short of what's needed to keep the temperature above freezing.

This doesn't definitively rule
out the greenhouse explanation; certain environmental processes the
researchers are not considering could have changed the composition
of the clays. Where does that leave things? It's largely the same
predicament as before. This study adds more evidence that Mars didn’t
have enough CO2 in its
atmosphere to have kept water liquid from 3.8 to 3.1 billion years ago.
So either warming was driven by some other mechanism, or somehow the
water was able to flow despite temperatures that were typically below
freezing. Either conclusion would be interesting.

The data also provides more information about a
critical period in the history of Mars—around the time these clays were
formed, Mars was rapidly losing its atmosphere. This site is among the
youngest of the now-dry sites of surface water yet discovered, meaning
these clays were probably formed during the tail end of this
process. Those were essentially the last days of the wet Mars.